Some shock absorbers mounted in a vehicle have a damping force adjustment mechanism. This mechanism may adjust a damping force generated by a piston because it is impossible to satisfy ride quality and operational stability, which are in a conflicting relationship, using a shock absorber in which the damping force is uniquely determined with respect to an operating speed of the piston. Various types of damping force adjustment mechanisms have been known. For example, JP 3103062 B (Reference 1) discloses “an adjustable vibration damper for a powered vehicle, which includes a cylinder having a damping fluid therein, a piston rod sealed and inserted into the cylinder and disposed to be movable in an axial direction, a damping piston fixed to the piston rod so as to divide the cylinder into two working chambers, and a damping valve having a valve seat and configured to adjust an effective cross section of a damping duct of a main stage by a valve body movable in the axial direction, characterized in that an axially movable valve element is disposed for the purpose of positioning of the valve body in one direction, the valve element biases a back side of the valve body by pressure of the damping fluid introduced from one working chamber through a flow communication portion having a throttle whereby the valve element generates a pilot control operation in this direction with respect to the valve body, and the valve body is directly biased in another direction, and the valve body is biased by the pressure of the damping fluid in the corresponding working chamber” (see claim 1 of Reference 1, but reference numerals are omitted).
JP 4985984 (Reference 2) is to “provide a damping force adjustable shock absorber capable of generating stable damping force even in the event of a failure of an actuator”, and proposes “a damping force adjustable shock absorber, which includes a cylinder having a fluid encapsulated therein, a piston provided to be slidable in the cylinder, a piston rod connected to the piston so as to extend to the outside from the cylinder, a passage in which a flow of the fluid is generated by a sliding movement of the piston in the cylinder, a pilot type damping valve configured to generate a damping force by controlling the flow of the fluid in the passage and adjust valve opening pressure with pilot pressure of a part of the flow of the fluid, a damping force adjusting valve configured to adjust the damping force by controlling a part of the flow of the fluid and adjusting the pilot pressure, and an actuator configured to operate the damping force adjusting valve, in which the damping force adjusting valve restricts the flow of the fluid in the event of a failure of the actuator, a relief valve is provided in parallel with the damping force adjusting valve, and a sub-damping valve configured to control the flow of the fluid is provided at a downstream side of the relief valve” (see Paragraphs [0010] and [0011] of Reference 2).
In the vibration damper disclosed in Reference 1, a mechanism capable of variably changing the damping force by operating the valve body by the actuator is disclosed. A control valve is disposed on the damping valve such that it is possible to control the pilot pressure of the flow of the fluid in one direction. However, the flow of the fluid in the reverse direction is controlled by directly operating the actuator. For this reason, in the event of a failure, it is necessary to set the damping force to any one of a maximum force and a minimum force. In contrast, in the case of adopting a structure including two damping valves, it is possible to control the pilot pressure of a bidirectional flow of the fluid, and as disclosed in Reference 2, the damping force characteristics in the event of a failure may be arbitrarily set. However, the number of components is increased and the size is enlarged with a complicated configuration, thereby causing a cost increase. In addition, a flow path is complicated, which makes it difficult to ensure stable failure characteristics. In particular, since a stroke amount is restricted due to the enlargement, when the shock absorber of Reference 2 is applied to a shock absorber for an automobile, stress is increased such that ride quality deteriorates.
Thus, a need exists for a damping force adjustment mechanism which is not susceptible to the drawback mentioned above.